In general, a multi-water heater is configured to have a capacity of a large-sized water heater by connecting a plurality of small-sized home water heaters in parallel. Everyone can manage the multi-water heater without a service man for water heater management unlike the large-sized water heater. Further, since the multi-water heater adopts the small-sized home water heater, the multi-water heater may be installed in a small space and easily maintained and managed. In addition, the multi-water heater is excellent in terms of energy saving because a plurality of water heaters are sequentially operated according to the amount of heat required for changing a temperature into a set temperature set by a user.
Sequentially operating the plurality of water heaters according to the amount of heat required that is changed according to the temperature set by the user is called a parallel operation.
In a method for controlling the parallel operation of the multi-water heater in the prior art, when each water heater is operated at a predetermined ratio (hereinafter, referred to as an ‘operation reference value’) or more of a capacity of each water heater, the stopped water heater is additionally operated and when each water heater is operated at a predetermined ratio (hereinafter, referred to as a ‘stop reference value’) or less of the capacity of each water heater, the operated water heater is additionally stopped.
However, when the operation reference value and the stop reference value are fixed at the time of operating the stopped water heater or stopping the operated water heater, an inefficient operation may be performed depending on the circumstance and conditions of use.
For example, when the water heater capacity is 48000 kcal/h, the operation reference value is 80% of the water heater capacity, and a temperature difference between a set temperature and a direct water temperature is approximately 13° C. (the set temperature is 43° C. and the direct water temperature is 30° C.), at least 49.2 liters of water per minute should be used as calculated below in order to additionally operate the stopped water heater.
            48000      ⁢                          ⁢      kcal      ⁢              /            ⁢      h              60      ⁢                          ⁢              min        ·        13            ⁢      °      ⁢                          ⁢              C        .              =      49.2    ⁢                  ⁢    l    ⁢          /        ⁢    min  
However, it is practically difficult that an amount of 49 liters of water flows through one water heater per minute. Accordingly, when the temperature difference between the set temperature and the direct water temperature is 13° C. and the operation reference value is 80% or more of the water heater capacity, the water heater is not additionally operated even though the stopped water heater needs to be additionally operated at the time of controlling the parallel operation of the multi-water heater, and as a result, the multi-water heater is inefficiently operated.